Summary

A previous fluorescence light-microscopic study showed that the fission yeast cold-sensitive beta-tubulin mutant nda3-311 was arrested with rod-like condensed chromosomes in a mitotic state at the restrictive temperature. Upon transfer to the permissive temperature, a spindle was formed and the nucleus was divided. In the present study, we employed freeze-substitution electron microscopy to examine the ultrastructure of arrested and released nda3-311 cells. In arrested cells, a single, displaced nucleus was seen with a single spindle pole body. Therefore, spindle pole body duplication seemed to require functional beta-tubulin. The nuclear membrane was highly deformed with a leaf-like profile in cross-section, possibly due to an interaction with the rod-like, condensed chromosomes. Upon transfer to the permissive temperature, the spindle pole duplicated and the daughter spindle pole bodies rapidly migrated to the opposite ends of the nucleus, accompanied by the formation of the mitotic spindle. Elongation of the nuclear envelope occurred with concomitant spindle extension, as in a wild-type mitosis. The deformed nuclear membrane became smooth and described a convex curve. The numerous vacuoles that are seen in the arrested cells decreased in number and increased in size. Septation was completed, leaving the two divided nuclei in one half of the cell. Hexagonally arranged microtubules, apparently forming the mitotic spindle, were observed in a cross-section of a cell after return to the permissive conditions.

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